A good sprinkling of salt on chips is for most of us like strawberries and cream. One without the other is immeasurably poorer and not quite the real thing.

But McDonald's, facing flak from the health lobby for its role in the obesity epidemic, will bow to public pressure this week and remove large quantities of salt from its products. Its rival Burger King is expected to follow suit shortly.

Removing salt from chips has not been an easy decision. Across the world customers have become used to eating their fries with a liberal dose of sodium chloride - and now they will be asked to refine their palates.

Nor is it just the fries that will change. The oil they are fried in will have less salt, as will the seasoning in the burgers and the ketchup that goes on them.

To measure the salt that goes onto a portion of fries, engineers at McDonald's have even devised a special shaker that will pour only a specified number of grains into the bag.

This will reduce the amount of salt in the portion by around 23 per cent, which in food industry terms is seen as a significant reduction. Last year Heinz removed only 11 per cent of the salt content from its tomato soup, because, it argued, any more would have changed the flavour significantly.

Health experts have called for a halving of the amount of salt in food over the next five years to save thousands of people from strokes, high blood pressure and early heart disease.

The country's leading salt expert, Professor Graham MacGregor of St George's Hospital in south London, welcomed the move last night. 'It's very good that they have done this, but I would question why they need to put salt on the chips at all. Why not leave them as they are and let customers sprinkle on what they want?

'Salt has been used for years by the food industry to disguise cheap, tasteless products. It's cheap and it makes you thirsty, so you buy more drinks, which also suits the industry.'

The Observer has learnt that the salt initiative comes after prolonged and difficult discussions between the UK branch of the junk food giant and its American headquarters in Oak Brook, Illinois.

A series of proposals for changes to the menu and ingredients were sent to the headquarters last year but met with resistance.

An insider said: 'The message came back that these were not to be tampered with, because it would change the flavour of our core products too much, and that customers wouldn't like it.

'We felt frustrated, because there's a sense that the Americans don't understand the pressure we are under from the health campaigners.

'We see the EC looking at fast food and thinking about regulation, and a lot of us feel that we need to start changing our products now.'

Responding to the claim that McDonald's will not change the fat and sugar levels, the spokeswoman said: 'We are constantly talking to the US about a range of proposals.

'We do have some core iconic products that are similar across the world, although there are some regional differences on menus.

'The whole obesity debate is a very complex one. It is far too simplistic to say that food is the answer, because it is about energy in and energy out.'

But the company, which has been losing market share in recent years and is now facing legal action over its role in the fat epidemic, has seen customers drift away to healthier alternatives.

Last week the company announced that it was ending supersizing, the policy of offering much larger portions for a small price difference.

It has also introduced salads to its menu, as well as a fruit option for children's meals and more healthy products will be announced this week.